Steering apparatuses of a so-called steer-by-wire type have been developed recently. Japanese Unexamined Patent Application Publication No. 2003-252227 discloses a steer-by-wire (SBW) vehicle steering apparatus.
In the SBW vehicle steering apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2003-252227, a clutch device that engages and disengages a column shaft and a transmission shaft is interposed therebetween. The column shaft rotates in accordance with the operation of a steering wheel, and the transmission shaft rotates in accordance with the operation of a steering mechanism. On the basis of the drive state (motor temperature or motor current) of a turning motor for turning a pair of turning wheels, a steering control unit determines whether the turning motor is in an overload state. If the turning motor is determined to be in an overload state, the steering control unit switches the clutch device from a disengaged state to an engaged state to mechanically transmit the operating force of the steering wheel to the steering mechanism, which performs steering.
The SBW vehicle steering apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2003-252227 can accurately suppress the occurrence of an overload in the turning motor.
In the SBW vehicle steering apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2003-252227, for example, if a temperature value of the turning motor continues to be higher than a predetermined threshold for a predetermined period of time, the turning motor is assumed to be in an overload state and the clutch device is switched from the disengaged state to the engaged state.
However, with the configuration described above, for example, in the event of an abnormality in a temperature sensor for the turning motor, the turning motor which is actually in an overload state may be erroneously diagnosed as not being in an overload state. In this case, the time of switching of the clutch device from the disengaged state to the engaged state may become later than the original switching time, and this may allow an overload of the turning motor.
There is a demand for an accurate diagnosis of whether a motor is in an overload state. Such a demand exists not only for the turning motor, but also for a steering reaction force motor.